In conventional drilling of bore holes for oil and gas recovery, there is often a necessity to isolate different types of formations with casing pipe to prevent cave-in of the bore hole walls and for loss of fluid, both during the drilling operation and recovery operation of the well. Currently, the predominant drill bit used by the well-drilling industry is the tri-cone rotary drill bit for oil and gas wells and water wells.
During a drilling operation, the size of the drill bit is many times limited by the inside diameter of the casing pipe previously introduced into the well. This requires a subsequent drill bit introduced into a well to have an exterior diameter somewhat smaller than the inside diameter of the previously introduced casing pipe, resulting in a bore hole having smaller diameters as the depth of the well proceeds. In drilling a typical well, a large diameter bit is routinely used to drill the upper portion of the hole to a first depth for a first casing string. A smaller diameter bit which will pass through the first casing string is then used to drill to a depth where a second casing string is introduced into the well. This continues until the depth of the well is achieved. In addition to requiring the use of various size bits with the attendant costs of drill bits and operational costs, the result is a bore hole having an effective diameter governed by the smallest diameter bit used in the drilling operation.
Heretofore, the drilling industry has utilized an eccentric bit, or a bi-center bit, to enlarge a bore hole below a tighter undersized segment, such as a previously introduced section of casing pipe. An eccentric bit includes an extended or enlarged cutting portion which, when the bit is rotated about its axis, produces an enlarged bore hold. A bi-center bit heretofore in use by the drilling industry employs two longitudinally spaced bit sections with laterally offset axes. The first axis is the center of the pass-through diameter, that is, the diameter of the smallest bore hole that the bi-center bit will pass through. The second axis is the axis of the hole as the bit is rotated.
Examples of bi-center bits are disclosed in U.S. Pat. Nos. 4,706,765 and 5,765,653.